One day it was raining outside and I was breathing on the glass of my window. I noticed that around the spots of fly poop there is a circle where vapor does not condense. With time the circle becomes larger and larger as the condensation slowly disappears.

Why does this happen?

Edit: Note that the poop is there from days ago, so it's not warmer than the glass. If the effect has something to do with the poop being warmer, please give me a motivation.

There is an image here which is also shown below. Sorry that it is out of focus; my phone's autofocus couldn't pick out the foreground image.

enter image description here

  • $\begingroup$ Do you have a picture of the phenomenon? $\endgroup$ Mar 22, 2014 at 16:34
  • $\begingroup$ @DumpsterDoofus I added it, take a look. $\endgroup$
    – HAL9000
    Mar 22, 2014 at 16:49
  • 2
    $\begingroup$ My guess is the poop has exuded a little bit of some fluid with very low surface energy. What you see in the "condenses" area is water droplets; in the poop circle, the water condenses but does not form a curved surface, so the light rays are not diffused. $\endgroup$ Mar 22, 2014 at 16:52
  • $\begingroup$ @CarlWitthoft I'm not a physician but I have a doubt: in that case why does the "dry" area grow? It should remain confined in the proximity of the source of low-surface-energy fluids, shouldn't it? $\endgroup$
    – HAL9000
    Mar 22, 2014 at 18:29

2 Answers 2


It's not poop. It's fly barf.


A fly spends about 25% of its time re-digesting and it only can eat liquids. It mixes the eaten food with the appropriate enzyme for digestion. The fly does this by retrieving the eaten food from its digestive system (a vomit of sorts), and drop by drop it is placed on the surface on which the fly is sitting. Only then is it sucked back up after they are mixed. The small black dots, that are left in various places, such as the ceiling, are not fly droppings, but actually the remains which are not sucked back up.

There are a variety of fluids that are mixed along with oils from the food. This creates a surface film much like when you write with your finger on a mirror. The growth of the spot is just the fluids slowly flattening out and spreading on the surface. The film prevents condensing water from beading to form a lens shape which defuses the light into the cloudy haze you see elsewhere. This is like an anti-fogging agent which works by minimizing surface tension, resulting in a non-scattering film of water instead of single droplets, an effect called wetting. Anti-fog treatments usually work either by application of a surfactant film, or by creating a hydrophilic surface.

The wetting may appear to spread near the edges of the film during evaporation because the droplets are on the cusp of being completely wetted and have more surface area to evaporate faster which gives the illusion of the spot spreading during evaporation.

As for the poop? Well, the droppings fall to the ground and go undetected.

  • $\begingroup$ +1. BTW, what are we seeing in the picture? Is it a fly? $\endgroup$
    – Řídící
    Mar 22, 2014 at 19:44
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    $\begingroup$ @GlenTheUdderboat yes that's the head of a common house fly (upside down) under magnification. $\endgroup$
    – user6972
    Mar 22, 2014 at 20:48
  • $\begingroup$ it makes enough sense for me so I'm checking this as good, thanks. $\endgroup$
    – HAL9000
    Mar 23, 2014 at 8:38
  • $\begingroup$ What is a good way to clean it off a roller curtain? ;-) $\endgroup$
    – my2cts
    Jan 9 at 8:28

I have an alternative explanation, as follows- and it might even be testable.

The fly vomit might be hygroscopic- that is, it wants to abstract water vapor from the air near it. in a moist environment where water wants to condense, the vomit depletes the humidity within some scale length by taking up water within that depletion zone, hence "stealing" it away so it cannot condense. This would produce a radially symmetric condensation-free zone with a diffuse edge.

Now if you gently blow moist air at the vomit glob you'll furnish moisture to the vomit faster that it can pull it out of solution and the condensate-free zone would disappear.

you can also place water droplets with a fine-bore syringe within the condensate-free zone and measure the resulting contact angle of the droplets with a goniometric microscope (which I used to construct for co-workers during my career in inkjet physics). if the vomit residue is poorly-wetting and has spread away from the blob, then the water will ball up near the blob like rain on a waxed car and spread out far from it.

wettability differences on a scale length of order ~tens of microns will perturb the trajectories of inkjet droplets as they emerge from nozzles in a surface possessing those wettability differences. In a previous lifetime, I had to routinely detect and photograph inhomogeneous wettability on injket printhead orifice plates using a metallographic microscope and a source of vapor-saturated air. I would love to try such a test on flyspecks!

  • $\begingroup$ An inkjet physicist, now I know why you were studying those globs. Not sure why the rest of were reading this though.😁 $\endgroup$ Jan 9 at 13:51

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